It is now understood that oxygenated aqueous solutions exposed to high energy gamma- or X-rays yield radiolytic products according to the following reaction: ##STR1## See, for example, Bors, et al., Curr. Top. Radiat. Res. 9:247 (1974). Energy-rich radicals, H.sup.. and e.sub.2.sup.-, lead to superoxide (.sup.- O-O.sup.. or O.sub.2.sup.-) formation at diffusion controlled rates (greater than 10.sup.10 M.sup.-1 s.sup.-1) in the presence of singlet or triplet state dioxygen according to the following reactions: EQU H.sup.. +.sup.. O-O.sup.. .fwdarw.HO-O.sup.. .fwdarw.H.sup.+ +.sup.- O-O.sup.. pH 4.7
e.sup.- +.sup.. O-O.sup.. .fwdarw..sup.- O-O.sup..
See, Behar, et al., J. Phys. Chem. 74:3209 (1970); Czapski, Ann. Rev. Phys. Chem. 22:171 (1971); Thomas, In "Radiation Research", Silini (ed.), Elsevier/North-Holland: New York, 1967, p. 179. Formation of superoxide partially accounts for the well known oxygen enhancement of radiation-induced cell damage.
The enzyme superoxide dismutase (SOD) plays a significant role in the defense against oxygen toxicity in aerobic organisms. Superoxide dismutase catalyzes the dismutation of O.sub.2.sup.-, as follows:. EQU 2 .sup.- O-O.sup.. +2H.sup.+ .fwdarw.H.sub.2 O.sub.2 +.sup.. O-O.sup..
See McCord, et al., J. Biol. Chem. 244:6049 (1969); Fridovich, Science 201:875 (1978). The rate of this reaction is also diffusion controlled, k =1.3.times.10.sup.9 M.sup.-1 s.sup.-1. Czapski, Ann. Rev. Phys. Chem. 22:71 (1971).
In a mammalian cell, two types of SOD are found. One contains both copper and zinc and is located in the cytosol and periplasmic space of the mitochondria (Cu-Zn SOD). The other enzyme contains manganese and is present in the matrix of the mitochondria (MnSOD). All normal mammalian cell types investigated contain these two types of the enzyme, except erythrocytes which lack MnSOD.
The specificity with which Cu-Zn SOD catalyzes the destruction of superoxide, together with the demonstrated radioprotection of enzymes, bacteriophage, bacteria, mycoplasma, and mammalian cells with Cu-Zn SOD, has prompted the examination of the prophylactic effect of Cu-Zn SOD on survival of whole-body 6.5 Gy (1 Gy=100 rads) x-irradiated mice. Petkau, et al., Biochem. Biophys. Res. Comm. 65:886 (1975); Petkau, et al., Biophys. Res. Comm. 67:1167 (1975); Petkau, et al., Int. J. Radiat. Biol. 29:297 (1976). The maximal effective dose of Cu-Zn SOD was 1.1 uM/kg. Higher and lower doses were less effective when the enzyme was given intravenously (iv) one hour prior to irradiation, the time at which maximum concentrations of .sup.125 I-labeled enzyme were found in bone marrow and bone marrow stem cells. Pretreatment with 1.1 uM/kg Cu-Zn SOD increased the LD.sub.50/30 dose of radiation from 6.3 Gy to 7.0 Gy and 10 percent survival was observed with a LD.sub.100/30 dose of radiation, 8.2 Gy. The same dose of Cu-Zn SOD given 1 hour before and 1 hour after irradiation further increased the LD.sub.50/30 dose to 8.7 Gy and 17 % of the mice survived when the radiation dose was increased to 10 Gy.
These results are consistent with the recognized normal biochemical role of Cu-Zn SOD and have been interpreted as explaining the well known "oxygen effect", the increase in radiation sensitivity associated with the presence of oxygen and the decrease in radiation sensitivity in the absence of oxygen.
Effective and less toxic radioprotectants are needed for protection of normal tissues of patients undergoing radiation therapy for neoplastic disease. The most effective radioprotectant developed to date, S-2-(3-aminopropylamino) ethyl-phosphorothioic acid (WR2721), produces a number of undesirable side effects. Ongoing clinical trials have been complicated by emesis, hypertension, hypotension, somnolence and allergy. Kligerman, et al., First Conference on Radioprotectors and Anticarcinogens, National Bureau of Standards, Gaithersburg, Maryland, June 21-24 (1982); Glick, et al., First Conference on Radioprotectors and Anticarcinogens, National Bureau of Standards, Gaithersburg, Maryland, June 21-24 (1982).
Concentrations of plasma copper complexes are known to increase in neoplastic disease states and to return to normal during remission. Sorenson, In "Copper Complexes in the Environment, Part II Health Effects", Niragu (ed.) John Wiley and Sons, New York, 1979, p. 83. Since copper complexes have antineoplastic, anticarcinogenic and antimutagenic effects, and since such complexes decrease in plasma concentration during remission, they may be a component of physiologic responses which facilitate remission.